On the validity of neutral gas temperature by N₂rovibrational spectroscopy in low-pressure inductively coupled plasmas

Abstract

Measurement of the rotational temperature of the second positive system of N 2 was used as a diagnostic of the gas temperature in low-pressure inductively coupled Ar, Kr and N 2 plasmas. The rotational temperatures determined from the rovibrational bands (ν , ν ) = (0, 0), (1, 0), (0, 2) and (4, 2) of the N 2 C 3 u → B 3 g system differ by about 300 K depending on the operating gas pressure in the 0.4-20 mTorr range. Important discrepancies exist between the temperatures found from each of the rovibrational bands of N 2 . This shows that the method has important intrinsic uncertainty that may be due either to errors in the transition probabilities of N 2 C 3 u → B 3 g or to inefficient thermal coupling between translational and rotational temperatures. In the case of argon, the population of the emitting C 3 u states by energy transfer from Ar 3 P 0,2 metastable atoms is also considered as a possible factor influencing the rotational structure of some rovibrational bands. Based on these measurements, it is shown that, in the range of experimental conditions studied herein, the uncertainty of the method should be carefully accounted before considering one of the rotational temperatures of the N 2 second positive system equal to the gas temperature.

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“…It is noted that the effects of adding molecular H 2 and N 2 gases to monoatomic rare gas Ar plasmas have so far been studied both experimentally and numerically/theoretically (see the Appendix). [107][108][109][110][111][112][113][114][115][116][117][118][119] In the present experiments, we did not measure the T e as well as electron energy distribution function (EEDF), and so we cannot say quantitatively the effects of adding a small amount (<1%) of H 2 and N 2 to Ar on the plasma properties. However, the present optical emission spectroscopy indicated that the addition of <1% H 2 and N 2 to Ar plasmas gave rise to no significant change of the Ar I line intensities in the visible and near-ir region (and thus no significant change of the intensity ratio of the visible Ar I lines to the near-ir ones); this implies that no significant change in the EEDF and/or T e occurred with the H 2 and N 2 addition, since the upper-level excitation energies for visible Ar I lines are >1 eV higher than those of near-ir ones (as mentioned above).…”

“…Moreover, the addition of N 2 may alter the plasma properties through a resonant electronic energy transfer between Ar metastable atoms and N 2 molecules [Ar*(3p 5 4s 3 P 0,2 ) + N 2 (X 1 Σ g + ) → Ar(3p 6 1 S 0 ) + N 2 (C 3 Π u )], 120,121 which can lead to an overestimation of the gas temperature derived from the vibronic spectrum of the N 2 2nd positive band assuming T g ≈ T rot . 115 On the other hand, when H 2 is added to Ar, a little different behavior occurs owing to a relatively large difference in magnitude of the ionization cross section by electron impact between Ar and H 2 [e.g., the total ionization cross section at E e = 50 eV is σ T /(πa 0 2 ) ≈ 2.88 for Ar, 2.20 for N 2 , and 1.07 H 2 , 122 where a 0 denotes the Bohr radius], and to a relatively large difference in mass of ion and neutral atoms/molecules between Ar and H 2 (e.g., the mass in amu or g/mol is M ≈ 39.94 for Ar, 28.01 for N 2 , and 2.015 for H 2 ). In these situations, with increasing the fraction of H 2 , the electron density n e tends to decrease, [107][108][109][110][111] the electron temperature T e tends to increase, [108][109][110][111] while the gas temperature T g does not vary significantly.…”

Neutral gas temperature estimates and metastable resonance energy transfer for argon-nitrogen discharges

“…Electrons having low mass do not significantly alter the molecular rotational moment during a collision and the ground states distribution effectively maps onto the upper state distribution. 10 A variety of rovibrational bands within a variety of molecules in various discharges have been used previously for gas temperature determination from rovibrational band fitting. The temperature of an atmospheric pressure air plasma was estimated using oxygen (O 2 ) and nitrogen ion (N þ 2 ) bands, and in agreement with each other within 2.4%.…”

“…20 The N 2 second positive system (C 3 P u ! B 3 P g ) is the most common band used, with gas temperature estimates made for capacitively coupled and inductively coupled plasma discharges, 5,10,[20][21][22] a radio-frequency (RF) helicon discharge, 23 and low pressure glow or positive column discharges. 24,25 Temperatures of higher pressure and atmospheric pressure discharges have also been analysed with the N 2 second positive system, 4,26,27 and also temperatures during the first 400 ns of spark breakdown for an N 2 discharge.…”

“…This has been previously done with trace amounts of N 2 in chlorine gas (Cl 2 ), 5,21 hydrogen gas (H 2 ), 28,29 and for noble gas discharges of helium 4,24 neon (Ne), 26 argon, 4,[8][9][10]16,22,23,30 and krypton (Kr). 10 Rovibrational band fitting to determine T g from T r of a diatomic molecule is only relevant if the single excitation channel to the upper measured state (here the C 3 P u state) is from electron impact. Being very low mass in comparison to N 2 molecules an electron collision has little effect on the rotational distribution, and the upper rotational distribution can be considered to be identical to the ground state rotational distribution, giving correct values for T r and hence T g .…”